# DP: Use /usr/include/<multiarch>/c++/4.x as the include directory
# DP: for host dependent c++ header files.

--- a/src/libstdc++-v3/include/Makefile.am
+++ b/src/libstdc++-v3/include/Makefile.am
@@ -1064,7 +1064,7 @@ endif
 
 host_srcdir = ${glibcxx_srcdir}/$(OS_INC_SRCDIR)
 host_builddir = ./${host_alias}/bits
-host_installdir = ${gxx_include_dir}/${host_alias}$(MULTISUBDIR)/bits
+host_installdir = $(if $(shell $(CC) -print-multiarch),/usr/include/$(shell $(filter-out -m%,$(CC)) -print-multiarch)/c++/$(notdir ${gxx_include_dir})$(MULTISUBDIR)/bits,${gxx_include_dir}/${default_host_alias}$(MULTISUBDIR)/bits)
 host_headers = \
 	${host_srcdir}/ctype_base.h \
 	${host_srcdir}/ctype_inline.h \
--- a/src/libstdc++-v3/include/Makefile.in
+++ b/src/libstdc++-v3/include/Makefile.in
@@ -1398,7 +1398,7 @@ parallel_headers = \
 @GLIBCXX_C_HEADERS_COMPATIBILITY_TRUE@c_compatibility_headers_extra = ${c_compatibility_headers}
 host_srcdir = ${glibcxx_srcdir}/$(OS_INC_SRCDIR)
 host_builddir = ./${host_alias}/bits
-host_installdir = ${gxx_include_dir}/${host_alias}$(MULTISUBDIR)/bits
+host_installdir = $(if $(shell $(CC) -print-multiarch),/usr/include/$(shell $(filter-out -m%,$(CC)) -print-multiarch)/c++/$(notdir ${gxx_include_dir})$(MULTISUBDIR)/bits,${gxx_include_dir}/${default_host_alias}$(MULTISUBDIR)/bits)
 host_headers = \
 	${host_srcdir}/ctype_base.h \
 	${host_srcdir}/ctype_inline.h \
--- a/src/gcc/Makefile.in
+++ b/src/gcc/Makefile.in
@@ -1279,6 +1279,7 @@ FLAGS_TO_PASS = \
 	"prefix=$(prefix)" \
 	"local_prefix=$(local_prefix)" \
 	"gxx_include_dir=$(gcc_gxx_include_dir)" \
+	"gxx_tool_include_dir=$(gcc_gxx_tool_include_dir)" \
 	"gxx_libcxx_include_dir=$(gcc_gxx_libcxx_include_dir)" \
 	"build_tooldir=$(build_tooldir)" \
 	"gcc_tooldir=$(gcc_tooldir)" \
@@ -2056,6 +2057,14 @@ gnat_install_lib = @gnat_install_lib@
 # host overrides
 -include $(xmake_file)
 
+# Directory in which the compiler finds target-dependent g++ includes.
+ifneq ($(call if_multiarch,non-empty),)
+  gcc_gxx_tool_include_dir = $(libsubdir)/$(libsubdir_to_prefix)include/$(MULTIARCH_DIRNAME)/c++/$(version)
+else
+  gcc_gxx_tool_include_dir = $(gcc_gxx_include_dir)/$(target_noncanonical)
+endif
+
+
 # all-tree.def includes all the tree.def files.
 all-tree.def: s-alltree; @true
 s-alltree: Makefile
@@ -3444,7 +3453,7 @@ PREPROCESSOR_DEFINES = \
   -DFIXED_INCLUDE_DIR=\"$(libsubdir)/include-fixed\" \
   -DGPLUSPLUS_INCLUDE_DIR=\"$(gcc_gxx_include_dir)\" \
   -DGPLUSPLUS_INCLUDE_DIR_ADD_SYSROOT=$(gcc_gxx_include_dir_add_sysroot) \
-  -DGPLUSPLUS_TOOL_INCLUDE_DIR=\"$(gcc_gxx_include_dir)/$(target_noncanonical)\" \
+  -DGPLUSPLUS_TOOL_INCLUDE_DIR=\"$(gcc_gxx_tool_include_dir)\" \
   -DGPLUSPLUS_BACKWARD_INCLUDE_DIR=\"$(gcc_gxx_include_dir)/backward\" \
   -DGPLUSPLUS_LIBCXX_INCLUDE_DIR=\"$(gcc_gxx_libcxx_include_dir)\" \
   -DGPLUSPLUS_LIBCXX_INCLUDE_DIR_ADD_SYSROOT=$(gcc_gxx_libcxx_include_dir_add_sysroot) \
--- a/src/gcc/cppdefault.cc
+++ b/src/gcc/cppdefault.cc
@@ -49,6 +49,8 @@ const struct default_include cpp_include
     /* Pick up GNU C++ target-dependent include files.  */
     { GPLUSPLUS_TOOL_INCLUDE_DIR, "G++", 1, 1,
       GPLUSPLUS_INCLUDE_DIR_ADD_SYSROOT, 1 },
+    { GPLUSPLUS_INCLUDE_DIR, "G++", 1, 1,
+      GPLUSPLUS_INCLUDE_DIR_ADD_SYSROOT, 2 },
 #endif
 #ifdef GPLUSPLUS_BACKWARD_INCLUDE_DIR
     /* Pick up GNU C++ backward and deprecated include files.  */
--- a/src/gcc/incpath.cc
+++ b/src/gcc/incpath.cc
@@ -164,6 +164,18 @@ add_standard_paths (const char *sysroot,
 			}
 		      str = reconcat (str, str, dir_separator_str,
 				      imultiarch, NULL);
+		      if (p->cplusplus && strstr (str, "/c++/"))
+			{
+			  char *suffix = strstr (str, "/c++/");
+			  *suffix++ = '\0';
+			  suffix = xstrdup (suffix);
+			  str = reconcat (str, str, dir_separator_str,
+					  imultiarch,
+					  dir_separator_str, suffix, NULL);
+			}
+		      else
+			str = reconcat (str, str, dir_separator_str,
+					imultiarch, NULL);
 		    }
 		  add_path (str, INC_SYSTEM, p->cxx_aware, false);
 		}
@@ -229,7 +241,16 @@ add_standard_paths (const char *sysroot,
 		  free (str);
 		  continue;
 		}
-	      str = reconcat (str, str, dir_separator_str, imultiarch, NULL);
+	      if (p->cplusplus && strstr (str, "/c++/"))
+		{
+		  char *suffix = strstr (str, "/c++/");
+		  *suffix++ = '\0';
+		  suffix = xstrdup (suffix);
+		  str = reconcat (str, str, dir_separator_str, imultiarch,
+				  dir_separator_str, suffix, NULL);
+		}
+	      else
+		str = reconcat (str, str, dir_separator_str, imultiarch, NULL);
 	    }
 
 	  add_path (str, INC_SYSTEM, p->cxx_aware, false);
